Direct Current power, also known as DC power, is popular for mobile device charging because it is easily stored in batteries.

All USB cables push DC power at up to 5 volts, but the amperage can vary. A standard USB-3 cable is rated to carry 900mA at 5 volts. Older USB-1 and -2 cables were rated for 500mA at 5 volts. Power Delivery (PD) cables, including Apple iPad chargers, push 2,100mA at 5 volts, while Kindle Fire chargers output 1,800mA at 5 volts. Many car chargers output 1,000-1,200mA at 5 volts.

USB charging ports, whether plugged into AC or DC power sources, can be rated up to 3,000mA. Don't be alarmed, though. There is no danger of damaging your USB cables or devices by plugging cables into more powerful ports. Any USB cable can safely be connected to any USB port. A higher-amp port may even charge your device faster, depending on its age.

USB technology limits cable length to 5 meters, but much shorter cables like the one included with your device are optimal for charging. Resistance increases over longer cable lengths, so extra-long USB cables will diminish voltage. Two amps run through a 2-meter USB cable diminishes the voltage by .77v if the cable uses a standard 28 gauge wire. Hardier cables with 24-gauge wire will lose only .3v over the same distance and amperage.Plug adaptors and connectors do not significantly impact the voltage.